1. Field of the Invention
The present invention relates to a cathode ray tube which enhances the uniformity of the brightness of an image display surface, and more particularly to a cathode ray tube which can make the brightness substantially uniform over the entire region of a panel portion by reducing the difference of light transmittivity between a center portion and a peripheral portion of the panel portion of the cathode ray tube which constitutes the image display surface and a manufacturing method thereof.
2. Description of the Related Art
Recently, cathode ray tubes which are called xe2x80x9cflat face typexe2x80x9d or xe2x80x9cplaner panel typexe2x80x9d have been popularly adopted as video tubes of television receiver sets or monitor tubes of personal computers or the like.
As panels which use glass as material thereof, a superclear panel, a clear panel, a semiclear panel, a gray panel, a tint panel, a dark tint panel and the like have been known in descending order of transparency. Currently, so-called semiclear panels have been popularly used in view of the reduction of the reflectivity of an external light by the panel per se and the reduction of the reflection of an external light due to phosphor coated on an inner surface of the panel.
Further, with respect to a related art which can suppress the reflection and the electrification of an outer surface, followings can be named. First of all, a cathode ray tube disclosed in Japanese Laid-open Patent Publication 345737/1992 is provided with a light selection/absorption layer between an inner surface of a panel portion and a phosphor layer. This light selection/absorption layer is a mixture made of more than two kinds of substances including dye or pigment made of organic compound or inorganic compound, wherein the particle size of the dye or the pigment is set to not more than 1.0 xcexcm and the light selection/absorption layer has two or more optical absorption peaks.
Further, on an outer surface of the panel portion, a mixture layer made of conductive material and binder, a single-layer reflection prevention film having the refractive index lower than that of glass which constitutes the panel portion, a multilayered reflection prevention film which is constituted of two to four layers having different refractive indices, or a film which mixes minute conductive particles made of ATO (Antimony Tin Oxide), ITO (Indium Tin Oxide) or the like in the multilayered reflection prevention film is formed.
Further, in Japanese Laid-open Patent Publication 182604/1993, a cathode ray tube which coats a coloring agent onto an outer surface of a panel portion and makes the density of the panel portion high at a center portion and low at a peripheral portion to make the light transmittivity of a panel uniform is disclosed. According to the invention disclosed in this publication, the coloring agent is mixed into a binder made of silica and is coated onto the outer surface of the panel portion by spraying and then a conductive agent which contains no coloring agent is sprayed onto the sprayed conductive agent so as to form irregularities on the surface of the panel portion. The glossiness (gloss value) derived from the irregularities on the surface can be adjusted by changing an amount of ethylene glycol added to the coating liquid.
Further, according to the invention disclosed in U.S. patent specification 4815821, a first transparent layer having the refractive index higher than that of panel glass is arranged to be brought into contact with an inner surface of a panel portion of a color cathode ray tube, an opaque pattern (light absorption matrix: black matrix (BM)) is formed on the first transparent layer, and a second transparent layer having the refractive index smaller than that of the first transparent layer is formed on the opaque pattern. The refractive index of the first transparent layer is set to 1.7 to 2.0 and the film thickness of respective transparent layers is set to xc2xc of the wavelength of a visible light.
The present invention can provide a flat-panel type cathode ray tube which exhibits a favorable flat feeling, enhances the uniformity of brightness over the whole screen, and exhibits the excellent contrast and the color reproductive range. Further, the present invention can provide a manufacturing method of flat-panel type cathode ray tubes which can reduce the manufacturing cost and can easily manufacture flat-panel type cathode ray tubes.
A typical constitution of a cathode ray tube according to the present invention includes a light transmission control layer which is formed of a mixture layer which is comprised of a first material containing particles which become transparent due to the irradiation of light or due to the irradiation of light and the oxidation on an outer surface of a panel portion which forms an image display surface and a second material containing particles which are chemically and physically stable and has the light absorbing ability and the conductivity, wherein the distribution of the transparent particles which constitute the first material has a gradation in which the number of the particles is small at a center portion of the panel portion and is continuously increased toward a peripheral portion of the panel portion.
Further, a low refractive index layer which has a refractive index lower than that of the light transmission control layer is formed as a layer above the light transmission control layer. The light transmission control layer is fixed to the panel portion, and a portion of the low refractive index layer is impregnated into the light transmission control layer to establish a stable physical and chemical bonding between the first material and the second material.
As a preferred example of the first material, silver, aluminum, halogen compound or silver sulfide is used. As a preferred example of the second material, precious metal (gold, platinum, silver or the like), nickel, chromium, titanium nitride, or a mixture of indium tin oxide (hereinafter referred to as xe2x80x9cITOxe2x80x9d) and a light absorption material is used.
Due to such a constitution, it becomes possible to obtain the substantially uniform brightness (luminance) over the whole region of the panel portion without providing the gradation of thickness between the light transmission control layer and the low refractive index layer formed on the outer surface of the panel portion. Further, when the light transmission control layer has the conductivity, the light transmission control layer functions as an electrification prevention layer and can also suppress the undesired electromagnetic radiation.
Further, a typical constitution of a manufacturing method of cathode ray tubes according to the present invention includes:
a step in which a first coating layer is formed such that a first dispersing liquid which is produced by mixing a first material which contains metal particles which become transparent due to the irradiation of light or the irradiation of light and the oxidation and a second material which contains metal particles or metal oxide particles which are chemically and physically stable, exhibit the light absorption ability and hold the conductivity into a solvent is coated onto an outer surface of a panel portion using a spin method,
a step in which a gradation exposure is applied to the panel portion such that the first coating layer is exposed in the oxidative atmosphere through an optical filter which exhibits the small transmittivity at the center of the panel portion and increases the transmittivity toward a periphery of the panel portion, and the metal particles which become transparent due to the irradiation of light or due to the irradiation of light and the oxidation are made transparent corresponding to an exposure light quantity which passes through the optical filter, and
a step in which a second dispersing liquid is coated onto the exposed first coating layer using a spin method, the first coating layer is fixed to the panel portion and the second coating layer which exhibits a refractive index lower than that of the first coating layer is formed and, thereafter, the second coating layer is baked.
As the second dispersing liquid, a hydrolysis liquid of silicon alkoxide which contains or does not contain a reducing agent can be preferably used. When the hydrolysis liquid contains the reducing agent, thiouric acid or hydroquinone is used as the reducing agent. Further, when the particles contained in the first material is metal, metal particles having the ionization tendency greater than that of the metal is used as the reducing agent.
As the first material, metal, halogen compound or metal sulfide is preferably used, while as the second material, the metal, metal oxide, metal nitride or a mixture of metal oxide and a light absorption material is preferably used.
As a preferred example of the first material, silver, aluminum or silver sulfide is named, while as a preferred example of the second material, precious metal (gold, platinum, silver or the like), nickel, chromium, titanium nitride or a mixture of ITO and a light absorption material is named.
Since the first material and the second material which are formed by lamination on the outer surface of the panel portion can be coated using the spin method, it becomes no more necessary to use the spin method together with a spray method and hence, the reduction of the manufacturing cost can be easily achieved by simplifying the manufacturing facilities.
As the matter of course, the manufacturing method of the present invention can use the spray method together with the spin method. That is, when an existing facility is provided with a spray coating device and the first material and the second material can be coated with a uniform thickness using such a spray coating device, it is unnecessary to introduce a spin coating device particularly. In such a case, if the spin coating device can be also used, the coating of uniform thickness can be achieved more easily.
Further, the present invention is not limited to the above-mentioned constitution and a flat-panel type cathode ray tube which will be explained in embodiments described hereinafter and is applicable to a cathode ray tube having a panel portion whose outer surface is formed of a curved surface or other similar image display device whose light transmittivity is different between a center portion and a peripheral portion in the similar manner.